New frequency spectrum allocations for personal communications have created a need for portable telephones which operate both in the existing 800 MHz frequency band as well as the newly allocated 1.9 GHz frequency band. Therefore, local oscillators for these dual band portable telephones are required to operate in two widely separated frequency ranges.
Prior art dual band devices have included; completely separate local oscillators with combiner networks, local oscillators with doublers or triplers to multiply the frequency, pin diodes or other similar RF switching devices, and extremely wide band oscillators having an operable frequency range that overlaps the two bands of interest.
The disadvantage of using completely separate local oscillators with combiner networks is that two complete sets of VCO circuitry are required in addition to the combiner network circuitry which adds cost and size to the overall circuit. Further, typical combiner networks waste power.
The disadvantage of using local oscillators with doublers or triplers to multiply the frequency is that spurious signals are always present in the output. These spurious signals must be filtered out to avoid degrading receiver performance or interference with other radio services. In addition, running multiple frequency sources wastes power.
The disadvantage of pin diodes is that pin diodes require significant DC current to obtain a low "on" impedance, and when the pin diodes are "off" they can create high levels of harmonically related spurious signals. Moreover, tank circuits associated with the pin diodes reduce circuit Q, which reduces efficiency, and causes higher phase noise in the output circuit.
The disadvantage of using extremely wide band oscillators is that wideband oscillators are necessarily very sensitive to tuning control. This sensitivity makes the oscillator more susceptible to noise on the tuning control line. Correspondingly, more sensitive tuning requires tighter coupling to the tuning (voltage variable reactance) element of the oscillator which causes higher losses in the associated tank circuit.
There is a need for a voltage controlled oscillator that: can generate different frequencies, operates on only one frequency at a time to save power, does not require completely separate oscillator circuits to obtain different frequencies, does not require combiners or pin diodes, has narrow band operation within either of two widely spaced frequency bands, is not sensitive to noise on a tuning control, exhibits a good frequency stability, minimizes spurious frequency signals, has low losses and current drain, and requires simpler, and therefore less costly, circuitry.